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Single Phase Capacitor motors.

  1. Mar 4, 2009 #1
    Hi guys, I have a question about working out the capacitance of a capacitor.

    I am not great at electronics, but if i understand correctly the capacitor in a capacitor motor is what makes it start spinning.

    I'm wondering is anyone could recommend a good resource for reading about how this works.

    I also have a problem that people may be able to help with. I have a motor that runs off 230 mains voltage and draws 1A. It has a horse power of 1/3 and has a speed of 2000rpm.

    Is it possible to work out the capacitance of the capacitor in this circuit to start the motor (if so how). If not what do i need to know to work this out.

    This is only a hypothetical question and i am not building a motor to work.

    Thanks guys,
  2. jcsd
  3. Mar 5, 2009 #2


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    The capacitance depends on alot of different things. Starting torque and line frequency would be the two big ones. Other things which would affect it would be the general design of the stator and rotor. Stator and rotor design will determine how much inductance that 'appears' to the line when 'looking into' the stator winding at zero RPM. Of course this all determines how much starting torque the motor has so like I said, the 2 big ones are starting torque and line frequency.
  4. Mar 5, 2009 #3
    In a single phase capacitor start induction motor, the capacitor in series with the starting coil provides a phase shift so that the current in the starting coil is out of phase with the current in the main coil. The less expensive split phase induction motors use a very resistive starting coil to shift the phase in the starting coil. The best fractional horsepower motors were the repulsion start induction motors, because the starting torque is high, and the starting surge current is low. My 1/8 HP repulsion-start motor has a patent date of 1915. These were relatively expensive motors, and were phased out when inexpensive start capacitors for FHP motors became available.

    I book I use is Audel's "Electric Motors" book, available thru Amazon or eBay.

    If your 230 volt motor runs at 2000 RPM, it is probably not an induction motor. Is it single phase or three phase? If it is three phase, it does not use a start capacitor. If you can get 1/3 HP out of 230 volts single phase at 1 amp, its efficiency would be over 100%.
  5. Mar 5, 2009 #4


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    Good point. I completely missed that.
  6. Mar 5, 2009 #5
    746 Watts per HP. Looks close to me.
    But where does the 2000 RMP come from?
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